Corona free spark gap



- Dec. 30, 1969 I T. E cul'hs 3,487,255

CORONA FREE SPARK GAP Filed Feb. 24. 1967 INVENTOR 7770/7745 5 CUP/7:6

BY j Z J I j ATToRwgs United States Patent Ofiice 3,487,255 Patented Dec. 30, 1969 3,487,255 CORONA FREE SPARK GAP Thomas E. Curtis, Centralia, Mo., assignor to A. B. Chance Company, Centralia, Mo., a corporation of Missouri Filed Feb. 24, 1967, Ser. No. 618,473 Int. Cl. H01j 1/52, 19/40 US. Cl. 313-313 8 Claims ABSTRACT OF THE DISCLOSURE During electrical storms, the transmission lines of electrical transmission systems may be subjected to impulse voltages induced by lightning which cause flashover at insulated support points. When this occurs, an impulse current first flows due to the impulse voltage; subsequently, the normal frequency voltage causes a follow current to flow along the ionized path of the preceding inpulse current. The follow current can be quite destructive to line insulators and the current carrying conductors. Tests have shown that, in the case of insulator strings utilized to suspend the conductors from tower crossarms, the lowermost insulator of the string is most vulnerable, the damage being particularly severe when the arc dwells on the hardware immediately under the string.

Protective gaps have been proposed heretofore for the purpose of shunting the arc path around the insulator unit in order to protect the latter from damage. However, the strike of an are inherently causes erosion of the gap surface resulting in pitting and point projections which render the gap structure subject to corona discharge during normal operation of the transmission line.

It is, therefore, the primary object of the invention to provide electric arc gap structure having a pair of spaced electrodes between which an arc may be struck and wherein at least one of the electrodes is of a configuration such that even though the same is deformed by the heat of the arc, it will not discharge corona at normal operating voltage following cessation of the arc.

Another important object of this invention is to provide a protective spark gap structure which is resistant to corona discharge after erosion of the gap conducting surface by the strike of an are.

As a corollary to the foregoing object, it is an important aim of the instant invention to provide such a spark gap structure which is resistant to corona discharge even though irregularities are present in the outer surface of the metal components of the structure.

Furthermore, it is an important object to provide a spark gap structure which confines the strike of the arc to a shielded zone.

In the drawing:

FIG. 1 is an elevational view of an insulator string and a conductor supported thereby, showing the instant invention mounted on the conductor;

FIG. 2 is a plan view of one of the gap elements;

FIG. 3 is a vertical sectional view taken along line 3-3 of FIG. 2, with the lower portion of the element broken away;

FIG. 4 is a plan view similar to FIG. 2, except that the element is shown after several arcs have struck the contact zone;

FIG. 5 is a vertical sectional view taken along line 55 of FIG. 4, with the lower portion of the element broken away;

FIG. 6 is a plan view of a spherical gap contact of the prior art; and

FIG. 7 is a plan view of the spherical contact shown in FIG. 6, illustrating the effects of arc erosion.

An outer end 10 of a metal crossarm of a transmission line support tower is illustrated in FIG. 1, an insulator string 12 being shown depending from the crossarm. The lowermost insulator 14 of string 12 is connected to a conductor clamp assembly 16 through which a currentcarrying conductor 18 of the transmission line extends. An armor rod 20 forms a sheath around conductor 18 in the region of clamp assembly 16, and may comprise a number of closely spaced aluminum rods wound around the conductor in a helix to protect the conductor against mechanical damage and possible arcs.

A pair of elongated, upstanding gap components 22 are secured to armor rod 18 on opposite sides of insulator 14 by a pair of mounting clamps 24. The right hand component 22 in FIG. 1 is shown in vertical, longitudinal section, revealing that each component 22 includes metallic inner structure 26 forming a core which is surrounded by a coating 28. Mounting clamp mechanism 24 is shown as being integral with structure 26 at the lower end of the latter but may be formed of separate, interconnected parts for manufacturing purposes if desired.

Referring particularly to FIGS. 2 and 3, the outer surface 30 of structure 26 has a generally frusto-conical, upwardly facing depression 32 therein, structure 26 being cylindrical in configuration but increasing in diameter at its upper end to form a bulk-like upper end portion presenting a circular rim 34 at the periphery of depression 32. Coating 28 covers all of surface 30 commencing just above mounting clamp 24, except for a circular surface zone 36 in the center of depression 32. Zone 36 forms the gap conducting surface to which the strike of the arc is confined, as will be explained hereinafter.

It is important that the coating 28 be less electrically conductive than the metallic structure 26, a variety of synthetic resins being suitable for use as the coating substance. However, if the machining of surface 30 to a smooth finish is to be avoided, it is requisite that coating 28 also have sufiicient electrical conductivity to distribute the electrostatic field around structure 26 (attributable to voltage normally on the conductor 18) in accordance with the contour of the exposed surface of the coating. The coating is applied while in the liquid state, such as by dipping a part of structure 26 in the liquid coating substance with zone 36 masked. The exposed surface of the coating after solidification is smooth and free of irregularities and thus distributes the electrical stress to preclude the establishment of point of high electrical stress by the irregularities in surface 30, which would be the case if the surface were uncoated and element 26 were not machined to a smooth surface finish.

In use, components 22 are spaced from insulator 14 such that the substantially rectilinear distance from point B of each zone 36 to point A of each insulator 14 and designated as L is less than the non-rectilinear distance from point A to point C and designated L as illustrated in FIG. 1. The pair of broken lines 38 represent shunt arc paths which, as illustrated, traverse a portion of the surface of the upper insulator 40 of string 12 and then bypass the remainder of the string and strike respective zones 36, whereupon the electric current is conducted to armor rod 20 and conductor 18 by structures 26, However, a pair of broken lines 42 represent another possible pair of are paths which traverse the insulator string until the lower insulator 14 is reached, whereupon each path represented by lines 42 extends to the adjacent gap contact zone 36, traversing a distance equal to L If it were not for the presence of components 22, the arc paths represented by lines 42 would continue along insulator string 12 across the lower portion of insulator 14, traversing a distance equal to L It Will be appreciated, therefore, since L is less than L the lowermost insulator 14 and clamp assembly 16 are not subjected to the arc and hence a dwell of the are in this area is prevented.

FIGS. 4 and 5 illustrate the erosion of contact zone 36 caused by several arc strikes. It may be noted that a void 44 is defined by irregular side and bottom surfaces of the interior or structure 26. Void 44 increases in size with each arc strike, thus the mass of structure 26 is larger at its upper end to increase the life of the structure.

By way of comparison, attention is directed to FIGS. 6 and 7 which show a prior art spherical gap contact before and after the effects of an impinging are. As would be expected, FIG. 7 reveals that the once smooth spherical surface is severely eroded, evidencing pitting and point projections. Therefore, the gap contact of FIGS. 6 and 7, once struck by an arc, is a source of corona discharge and must be replaced if corona problems are to be avoided during normal operation of the transmission line.

In the instant invention, however, the annular portion of structure 26 presenting rim 34 effectively shields zone 36 and renders the gap structure resistant to corona discharge. It will be appreciated that, although pitting and point projection inherently occur as illustrated in FIGS. 4 and 5, this erosion is deep within the depression 32.

Coating 28 cooperates with the structural configuration discussed above in resisting corona. The coating serves a two-fold purpose. First, being less conductive than structure 26, the arc strikes the exposed surface zone 36 within depression 32 rather than rim 34 or other regions of component 22. Secondly, since coating 28 is slightly conductive and presents a smooth, exposed surface as discussed above, irregularities in the outer surface 30 of structure 26 do not contribute to the production of corona discharge. Coating materials for rendering irregular surfaces resistant to corona discharge are discussed in detail in a co-pending application of Pink et al., Ser. No. 509,892, filed Nov, 26, 1965, now abandoned, entitled Corona Discharge Resistant Electrical Transmission Hardware, and owned by the assignee herein.

Examples of synthetic resin substances suitable for use as the coating material are as follows:

(1) A plastisol composed of a finely divided dispersion of polyvinyl chloride and a plasticizer such as dioctyl phthalate or 2-ethyl-hexylphthalate (2) Neoprene synthetic rubber (3) Epoxy resin (4) Polyester resins (5) Polyolefins (6) Urethane elastorners Most synthetic resins require the addition of a conductive additive to impart the requisite conductive characteristics to the substance. Carbon black is a particularly useful additive in this respect since one to two percent carbon black by weight provides the resin with resistance to ultraviolet light and, in so doing, renders it slightly conductive, having sufiiciently low resistivity to effectively electrically connect the outer surface of the coating with the underlying metallic surface.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A corona free flashover protection device for an insulator unit and a current-carrying conductor supported thereby, said device comprising:

electrically conductive electrode structure provided with an outer surface having a depression therein;

means coupled with said structure for electrically connecting the latter to said conductor, and for positioning the structure with said depression facing away from said conductor and establishing a spark gap defining an arc path shunting at least a portion of said unit;

said depression having a surface zone therewithin defining a gap contact lying in said path; and a cover material on at least a part of said outer surface exclusive of said zone and surrounding the latter;

said material being less electrically conductive than said structure, whereby to confine the strike of an arc, and hence the resultant erosion of the element, to said zone.

2. The invention of claim 1,

said material comprising a coating adhered to said outer surface exclusive of said zone and provided With a smoothly contoured, exposed surface;

said coating having sufficient electrical conductivity to distribute an electrostatic field around the structure induced by the voltage normally impressed on said conductor in accordance with the contour of said exposed surface, whereby to prevent irregularities in the underlying outer surface of the structure coated with said material from inducing corona discharge.

3. The invention of claim 2, said coating including a synthetic resin substance bonded to said underlying outer surface.

4. The invention of claim 1,

said depression being of generally frusto-conical configuration;

said zone being centered in said depression.

-5. The invention of claim 1,

said depression defining a peripheral rim spaced from said zone;

said material extending over said rim and into the depression to said zone.

6. The invention of claim 1, said means including clamp mechanism for mounting said structure on said conductor adjacent said unit.

7. The invention of claim 1,

said structure being elongated in configuration and having a pair of opposed ends;

said depression being disposed in one of said ends;

said means including clamp mechanism for securing the other of said ends to said conductor with the structure extending therefrom adjacent said unit.

8. The invention of claim 1,

there being a pair of said structures;

said means including clamp mechanism for mounting said structure on said conductor on opposite sides of said unit.

References Cited UNITED STATES PATENTS 1,552,664 9/1925 Austin 313-313 X 1,906,602 5/1933 Hull 313-313 X 2,135,352 11/1938 Rorden 313-313 X 2,135,353 11/1938 Rorden 313-313 X FOREIGN PATENTS 371,471 4/ 1932 Great Britain.

JOHN W. HOCKERT, Primary Examiner J. R. SHEWMAKER, Assistant Examiner U.S. Cl. X.R. 313-326; 317-72 

